For numerous industrial decoration tasks, for example the decorative printing of floors and furniture surfaces, the production of classic print media, in packaging printing, but also in so-called functional printing like the creation of printed circuits, solar cells, bio-chips etc., high-resolution industrial ink-jet printers are replacing classic printing methods such as offset, gravure and screen printing.
Within the framework of this specification, the term “ink” and “ink-jet printing” are to be understood in the most general sense. While in the production of graphic finished products such as posters, printed packaging etc. ink in the narrower sense is actually ejected through the print heads in the form of minute drops onto the substrate to be printed, such as paper, foil, cardboard, textiles etc. and designs these in color, in so-called “functional” ink-jet printing, special fluids are also ejected onto a substrate using basically the same principle in the form of minute drops in order to create a chemo-physical function on this substrate: argentiferous fluids to create printed conductors, molecular-biologically active fluids to create so-called bio-chips, semi-conductor fluids to “print” screens etc. All of these processes are often referred to under the vague term of “digital printing”.
At least in the industrial sector, this so-called “digital printing” uses mainly piezoceramic, so-called drop-on-demand, print heads, in which through piezoelectrically generated shear and/or compressive forces minute ink drops of typically 10 picoliters per drop are ejected onto the substrate to be printed through a large number of closely adjoining nozzles with repetition rates of up to 20 kHz.
Besides the undisputed advantage of the more or less direct transmission of an electronically saved file onto a physical carrier and the associated option of printing very small batches etc., however, a basic weak point remains. Through the extremely high number of nozzle switching operations per unit area, for example approximately 100 million per square meter on a furniture panel to be decorated, the probability of the temporary or complete failure of a nozzle is not negligible.
Typical nozzle faults are nozzles blocked by dirt in the ink, sedimentation or air bubbles, nozzles that do not close properly or nozzles that function irregularly. While numerous new developments like the so-called “side-shooter” nozzle heads by the company Xaar (www.xaar.com) reduce the likelihood of such malfunctions, they cannot rule them out completely. The problem of nozzle failure is described in, among other articles, Chry Lynn: “Drops and Spots: Latest Trends in Inkjet Printheads and Printer Design”; SGIA Journal, 4th quarter 2009, pp. 14-17.
Since technological developments are moving towards higher and higher-resolution print heads with higher and higher switching frequencies, this inherent problem will increase and hinder the further propagation of a cost-effective and technically highly interesting technology.
Very early in the history of the development of ink-jet printers for digital printing, there were efforts to monitor the correct functioning of ink-jet printers.
Basically, this monitoring can take place on two levels:    a) the monitoring of each individual ink ejection nozzle for correct drop ejection by means of a sensor, as a rule contactless, and    b) the monitoring of the printed result, as a rule through the image-generating recording of the printed substrate (paper, wood panel, solar cell glass etc.) in a camera-based process.
As early as 1991, the company Siemens AG, Munich, had described a process in WO 91/00807 in which the ejection of the (warm) ink drop from the nozzle was contactlessly detected with the help of a thermal sensor.
The U.S. Pat. No. 6,350,006 also teaches how the optical density of the ink curtain formed by the drop ejection is monitored with the help of photosensors.
In its large industrial ink-jet printer HPT300 Color Inkjet Web Press, the company Hewlett-Packard uses its own camera-based image processing system, which records a test sample printed at periodic intervals and in this way detects nozzle faults.
As a rule, the effect of the recognition of a malfunction in an ink-jet print head is to stop current production and to service/clean the print heads affected. There is no doubt that stopping production temporarily in this way reduces productivity significantly and is thus very expensive.
In addition, there have been a number of proposals to minimize the visual effect of unavoidable printing faults, that is, in the event of a printing fault to take measures to minimize the visual effect of the non-functioning nozzles without stopping production.
For example, the U.S. Pat. No. 6,786,568 B2 describes a method of printing over faulty areas with a special ink with the help of a number of additional nozzles in order to cover up optical detection. A precondition for this, however, is a sufficiently robust detection of faulty nozzle functioning.
The dissertation by Jia W I E “Silicon MEMS for Detection of Liquid and Solid Fronts”, T U Delft, 13 Jul. 2010, Chapter 4: “Liquid Surface Position Detection for Inkjet Meniscus Monitoring” also describes how the correct formation of the ink meniscus can be monitored capacitively with the help of extremely miniaturized sensors within an ink-jet nozzle.
Despite these prior-art methods for monitoring the individual nozzles of an ink-jet printer, ink-jet print heads are seldom supplied and used with an integrated individual nozzle monitoring system today due to the unreliability and complexity of these additional monitoring organs. End customers make do with frequent printing and the evaluation of test samples and have so far accepted the associated production downtimes.